Human ehrlichiosis is caused by three agents including Ehrlichia chaffeensis, the human granulocytic ehrlichiosis agent and by E. ewengii. These potentially fatal infections pose a serious threat to public health, particularly to immunocompromised and elderly people. However, little is known about the mechanisms of host resistance and reasons for the emergence of these diseases. We propose to characterize the host immune response necessary for clearance using mouse as the experimental host because the course of E. chaffeensis infection is similar between immunocompetent humans and mice and the model will provide data for human applications. Moreover, the extensive array of molecular, immunological and genetic tools available for mice will allow us to manipulate the system in ways that are not possible in other species. Specifically, we propose to test three hypotheses: 1) TH1 type helper T-cells are required for clearance of E. chaffeensis; 2) Secreted cytokines from T-cells are needed for host resistance to E. chaffeensis infections; 3) Macrophage activation contributes to the clearance of E. chaffeensis. Several observations form the basis for this project: immunocompetent mice clear E. chaffeensis by 16 days and the clearance is associated with the expression of an E. chaffeensis-specific TH1 type IgG response, CTL response and granuloma formation; the infections are fatal in SCID mice lacking T- and B-cells; mice lacking functional MHCII genes establish long-term persistent infections after E. chaffeensis challenge; mice deficient for macrophage activation develop short-term persistent infections; and prior activation with IFN-gamma inhibits monocyte infections with E. chaffeensis. We will evaluate the importance of helper T-cells, cytotoxic T-cells, B-cell responses, cytokines, and macrophage activation using several mouse strains with different genetic backgrounds and by manipulating their immune systems to diminish or enhance particular immune components. Infections will be monitored using bacteriological, molecular, immunological, and pathological analyses to assess host immunity and infection status. The long-term goals of this project are to elucidate host immune mechanisms, pathogen evasion strategies and to ultimately use the information to devise effective intervention measures against E. chaffeensis and other closely related organisms.